using namespace std;

class Solution {
public:
	int countNumbersWithUniqueDigits(int n) {
		if (n == 0) {
			return 1;
		}

		if (n == 1) {
			return 10;
		}

		int ans = 10, cur = 9;
		for (int i = 0; i < n - 1; i++) {
			cur *= 9 - i;
			ans += cur;
		}

		return ans;
	}

};